2024 – Implications of Large Changes in National Seismic Hazard Updates in Australia in 2018/2023 and New Zealand in 2022

Paul Somerville, Roseanne Coulter

Recent updates of national seismic hazard models in Australia in 2018 and 2023 (NSHA18 and NSHA23) approximately halved seismic hazard levels while the update in New Zealand in 2022 (NSHM22) approximately doubled them in many locations. Changes in building code levels are usually much smaller, on the order of ten percent, and even changes at those levels can be significant for design. This raises questions of why the changes were so large, why they changed in opposite directions, and whether such large changes could occur again. The large reductions in hazard levels in Australia in 2018, partially offset in 2023, are mainly based on the correction of earthquake magnitudes in the earthquake catalogue and their conversion to moment magnitudes Mw. Future large changes in these magnitudes are not expected, so the NSHA hazard results from future updates of earthquake magnitudes are not expected to change greatly. The largest sources of change are expected to be from changes in ground motion models because of the recent rapid acquisition of strong motion recordings in Australia, but these changes are not expected to be large. Another source of change could come from changes in fault slip rates, which would only have a large effect on ground motion levels at sites in Australia located close to faults.

The large increases in hazard levels in New Zealand in 2022 are mainly attributable to the use of a new methodology in the modelling of earthquakes on faults (Field et al., 2014), which has undergone two revisions in California where it was first applied, and thus may also be subject to change in New Zealand. That new method is not very relevant in Australia, where faults make a smaller contribution to the seismic hazard than distributed seismic sources based on historical seismicity except at locations close to faults. In many cases, the slip rates of the faults are quite uncertain, causing uncertainty in the resulting seismic hazard level. The uncertainty in hazard level due to slip rate uncertainty is expected to be larger than that due to the uncertainty in the historical seismicity rate embodied in the distributed seismic sources, because the historical seismicity builds gradually with time while new fault slip rate information can appear abruptly. Because fault sources contribute more to the total seismic hazard in New Zealand than in Australia, more uncertainty in seismic hazard estimation is expected in New Zealand that Australia. Another cause of the large increases in hazard levels in New Zealand is the use, not present in NSHM10, of shallow crustal earthquake models with additional epistemic uncertainty, which increases ground motion levels at long
return periods. It also uses newly developed ground motion models in which changes may occur when they have been subjected to more scrutiny.